This invention relates to a tungsten sealing glass to be used for a glass tube in a fluorescent lamp which serves as a light source of a lighting equipment for a liquid crystal display device or the like.
Liquid crystal display devices are broadly classified, depending upon manners for utilizing light sources, into a reflection-type of liquid crystal display devices using natural light or light from room lighting, and a transmission-type of liquid crystal display devices using light from a dedicated lighting equipment, for example, a backlight device. For those applications, such as notebook-type personal computers, TV monitors, and in-vehicle instruments or indicators, which require a high-quality display, the transmission-type liquid crystal display devices with the backlight device are mainly used. For wristwatches, small-sized electronic desk calculators, and the like which are of the type especially low in power consumption, the reflection-type liquid crystal display devices are used. Recently, there are, however, some devices of the low power consumption type which have a frontlight device used by being turned on whenever needed.
The principle of emission of a fluorescent lamp serving as the light source of the backlight device or the frontlight device is similar to that of an ordinary fluorescent lamp for lighting. Specifically, a mercury gas, a xenon gas, or the like enclosed therein is excited by discharge between electrodes so that the excited gas radiates ultraviolet rays to make a phosphor applied on the inner wall surface of a glass tube emit visible light. However, the fluorescent lamp serving as the light source of the backlight device or the frontlight device is greatly different from the ordinary fluorescent lamp in that the diameter of the glass tube is small and the wall thickness thereof is thin.
Conventionally, for the glass tube of the fluorescent lamp of this type, use has been made of a lead-soda soft glass because of easiness in workability and long-period achievements as a glass for illumination. As seal-in metal, use has been made of Dumet which is inexpensive.
As the liquid crystal display devices become smaller in thickness, lighter in weight, and lower in power consumption, the fluorescent lamp is also required to be smaller in diameter and thinner in wall thickness. However, the smaller diameter of the fluorescent lamp is structurally accompanied by a decrease in mechanical strength and an increase in heat generation of the lamp so that the glass tube is required to be high in strength and low in expansion. Further, in order to improve a luminous efficiency, development is made of a lighting circuit operated at a higher frequency. Consequently, the glass tube as an insulator is required to be high in volume resistivity and low in dielectric loss. Therefore, it is no longer possible for the conventional lead-soda soft glass material to satisfy the above-mentioned requirements.
In view of the above, consideration has been made of production of the fluorescent lamps by the use of a borosilicate hard glass which is high in thermal and mechanical strengths and advantageous in respect of electrical insulation in comparison with the lead-soda soft glass. As a result, a fluorescent lamp has been developed and commercialized which use a tungsten sealing glass and a tungsten metal already known as a combination of a hermetically sealable hard glass and a metal.
However, the above-mentioned glass tube of the fluorescent lamp for the backlight device is produced by directly using conventional tungsten sealing glass of a borosilicate material which has been generally used for a xenon flash lamp and simply by shaping and processing the material into a narrow tube, and therefore, has the following problems (i) through (iv).
(i) The glass is discolored by ultraviolet rays radiated from the excited mercury gas or the like (so-called ultraviolet solarization) of the glass. The discoloration of the glass causes decrease in brightness or deviation in luminous colors, leading to deterioration in quality of the liquid crystal display device.
(ii) The glass is a diversion of one originally intended to use for the xenon flash lamp and is, therefore, designed so as to allow transmission of a certain amount of ultraviolet rays in order to endure flashing of the xenon flash lamp. However, in case of use for the fluorescent lamp, such transmission of the ultraviolet rays causes discoloration and deterioration of other component parts in the backlight device or the frontlight device, for example, a light-guiding plate and a reflection plate made of resin.
(iii) Due to extremely high devitrification, the glass is liable to be devitrified and deteriorated during formation of the tube glass. It is therefore difficult to produce the glass tube with high dimensional accuracy. When the glass tube poor in dimensional accuracy is used, uniform application of the phosphor is impossible, resulting in nonuniform brightness. Moreover, in an optical system comprising the fluorescent lamp, the light-guiding plate, and the reflection plate, it is impossible to assemble these component parts together exactly in conformity with designed dimensions. This results in a decrease or a nonuniformity in brightness of a backlight device unit or a frontlight device unit itself.
(iv) Since the volume resistivity of the glass at 250xc2x0 C. is about 108.5 xcexa9xc2x7cm, electrical insulation is not sufficient. In case where the fluorescent lamp has a small diameter, a long size, and a high brightness, a voltage to be applied for lighting is high and reaches several hundred volts. However, in a conventional glass which is not high in electrical insulation, a leakage occurs to cause heat generation. In the worst case, the glass may be melted so that the function as the lamp will be lost completely.
It is therefore an object of the present invention to provide a tungsten sealing glass which is excellent in ultraviolet solarization resistance, ultraviolet shielding, devitrification, and electrical insulation and is therefore suitable for a glass tube of a fluorescent lamp for use in a backlight device or a frontlight device.
According to the present invention, there is provided a tungsten sealing glass for use in a fluorescent lamp, which has a composition of, by mass percent, 65-76% SiO2, 10-25% B2O3, 2-6% Al2O3, 0.5-5.8% MgO+CaO+SrO+BaO+ZnO, 3-8% Li2O+Na2O+K2O, 0.01-4% Fe2O3+CeO2, 0-10% TiO2+Sb2O3+PbO, and 0-2% ZrO2, where Na2O/(Na2O+K2O)xe2x89xa60.6.